Dr. Jeff Hersh: Watch out for jellyfish

Tuesday

Apr 27, 2010 at 12:01 AMApr 27, 2010 at 6:03 AM

Several of the more than 200 types (and 10,000 species) of jellyfish-like creatures can be found in every ocean. Of the 10,000 species of jellyfish-like creatures, about 100 have toxin that can affect humans. Thankfully, these usually cause only skin reactions.

Dr. Jeff Hersh

Q: Are there really invisible jellyfish that can kill someone?

A: Several of the more than 200 types (and 10,000 species) of jellyfish-like creatures can be found in every ocean. There are many differences between these gelatinous creatures, but in this overview I will concentrate on the similarities and take some liberties making generalizations.

Jellyfish are 90 percent water (people are about 67 percent), and are typically translucent (not invisible). They have an umbrella-like structure (where the mouth and digestive system are), and tentacles that hang down from it. The size of the umbrella varies quite a bit, from less than an inch to several feet or more. The tentacles can be inches long but in some species can be over 100 feet.

Jellyfish are hunters. Some just float along, possibly hitching a ride on seaweed, and some propel themselves (the fastest swim about 10 feet a minute). Clearly even these "swimmers" cannot overtake their prey with speed; instead they are "passive" hunters, utilizing their tentacles (which are covered with toxin-filled sacs called nematocysts) to kill.

A jellyfish senses mechanical and/or chemical stimulation when unsuspecting prey wanders too close, and then uncoils small skin-piercing needle-like organelles that inject toxin. Jellyfish cannot hold their catch if it fights back, so their toxin must be fast-acting.

Of the 10,000 species of jellyfish-like creatures, about 100 have toxin that can affect humans. Thankfully, these usually cause only skin reactions. However, the Portuguese man-of-war, the box jellyfish and the Irukandji jellyfish can be lethal to humans. These are usually encountered in the Indo-Pacific areas, especially around Northern Australia.

The man-of-war is actually a bluish-purple colony of four different animals acting together, not a jellyfish. However, it is gelatinous, has an umbrella-like structure (although its umbrella fills with a gas and acts as a sail) and has tentacles (which average 30 feet long, but can be 100 feet or more). Its sting may cause painful hives and skin irritation, but there have been human deaths from it.

The umbrella of the box jellyfish (also called the sea wasp or marine stinger) is boxlike in shape (hence the name) and inches across (the tentacles are a few feet long), although some species can weigh up to 4 pounds (with tentacles 10 feet or longer). Although many species of jellyfish can sense light, some species of box jellyfish have true eyes and can "see," to some extent. The sting of the box jellyfish may just cause pain, but has been implicated in almost 100 deaths.

This brings us to the little killer, the Irukandji jellyfish. Its body is an inch or smaller, with tentacles a couple of inches or less, making it difficult to detect. Its "discovery" illustrates this point. Some swimmers had developed Irukandji syndrome (back/muscle pain/cramps, burning skin sensations, headache, nausea, vomiting, sweating, increased heart rate, increased blood pressure, extreme anxiety, and/or fluid build-up in the lungs) and several had died from it, but the cause had not been identified.

In 1964, Dr. Jack Barnes suspected these symptoms were from an unidentified jellyfish sting, and he set out to discover the culprit. As he lay in the water, a tiny jellyfish floated past his facemask. He captured it and stung himself, his son and a lifeguard. All three came down with Irukandji syndrome; thankfully all three survived.

Here are some jellyfish safety tips:

- Never swim alone.

- Check with local lifeguards about jellyfish concerns in the area.

- If you are stung, seek immediate first aid. Box and possibly Irukandji jellyfish stings should be treated with vinegar to prevent toxin release from remaining nematocysts (some man-of-war nematocysts may release toxin from vinegar treatment, so it is no longer recommended for their sting). Remaining nematocysts should be removed (the rescuer should not use their bare hands so they don't get stung). Symptomatic treatment, including antihistamines, pain medications and local treatments such as ice packs, should be initiated.

- People with severe reactions may need to be hospitalized. Aggressive supportive care and treatment for severe allergic reactions may be needed, and there is anti-venom for box jellyfish stings. Some recent research has shown that intravenous magnesium therapy may be helpful for severe Irukandji stings.

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